In a direct injection fuel injector of an internal combustion engine for an automobile, atomization of the injected fuel and reduction of penetration of the injected fuel are desired in order to suppress adhesion of fuel to the cylinder wall surface and the piston crown surface. JP2010-248919A discloses a method of promoting atomization of fuel. According to this method, a diffuser portion consisting of an increased diameter portion is formed in an injection hole such that an outlet end of the injection hole is greater in diameter than the inlet end thereof, and separation vortices of a fuel flow are created in the injection hole.
However, in view of further improving thermal efficiency and minimizing impacts on the environment, further atomization of fuel and further reduction in penetration are desired.
In view of such problems of the prior art, a primary object of the present invention is to provide a fuel injector that allows further atomization of fuel and further reduction in penetration.
To achieve such an object, the present invention provides a fuel injector, comprising: a nozzle (21) including a tubular nozzle main body (27) extending along a predetermined central nozzle axial line (X) and internally defining a fuel passage (26), and a nozzle tip portion (28) including a bottom wall (30) defining an annular valve seat (29) facing the fuel passage in a coaxial relationship to the central nozzle axial line, the nozzle tip portion being provided with a plurality of injection holes (35) passed through the bottom wall and surrounded by the annular valve seat; and a valve member (23) disposed in the fuel passage to be moveable along the central nozzle axial line and configured to be selectively seated on the valve seat; wherein at least one of the injection holes includes an inner hole section (71), a middle hole section (72) and an outer hole section (73) in that order from a side of the fuel passage, the inner hole section extending from an inner surface (60) of the bottom wall obliquely away from a first side relative to a normal line of the inner surface of the bottom wall so as to define a first inner side wall surface (81) on the first side forming an obtuse angle relative to the inner surface on the first side and a second inner side wall surface (82) on a second side opposite to the first side forming an acute angle relative to the inner surface on the second side, the middle hole section including a first middle side wall surface (83) connected to the first inner side wall surface so as to extend obliquely relative to the first inner side wall surface toward the first side, and the outer hole section including a first outer side wall surface (85) connected to the first middle side wall surface so as to extend obliquely relative to the first middle side wall surface toward the first side; wherein a recess (65; 89) is formed on a radially outer side of an inner end of the inner hole section with respect to the central nozzle axial line and/or a part of the valve member opposing the radially outer side of the inner end of the inner hole section with respect to the central nozzle axial line.
Thereby, the fuel ejected from the injection hole can be further atomized, and can be limited in penetration. As the valve member is lifted from the valve seat, part of the fuel flows from a radially outer direction into the inner hole section, and the recess increases the sectional area of this flow, causing a reduction in the velocity of the fuel flow in this region. In addition, the inner side wall surface on the second side forms an acute angle relative to the inner surface of the bottom wall. Therefore, the part of the fuel flow entering the inner hole section from the second side separates from the inner side wall surface on the second side immediately after entering the inner hole section, and resulting turbulence promotes the atomization of the fuel. Further, because the middle side wall surface on the first side inclines away from the second side, the fuel flow advancing into the middle hole section is prevented from colliding with the middle side wall surface on the first side. Similarly, because the outer side wall surface on the first side inclines away from the second side, the fuel flow advancing into the outer hole section is prevented from colliding with the outer side wall surface on the first side. Thereby, the cone of the fuel flow is prevented from being narrowed. Owing to these features, the atomization of the fuel is enhanced, and the penetration of the fuel flow is minimized.
In this invention, it is preferable that the middle hole section includes a second middle side wall surface (84) on the second side continuously extending from the second inner side wall surface in a same direction.
Thereby, the drilling work for the injection hole is simplified while ensuring a favorable separation of the fuel flow from the side wall surface on the second side.
In the above configuration, it is preferable that the outer hole section includes a second outer side wall surface (86) on the second side that extends obliquely from the second middle side wall surface toward the second side for a short distance from the second middle side wall surface, and thence extends in parallel with the second middle side wall surface.
This feature also facilitates the drilling work of the injection hole while ensuring a favorable separation of the fuel flow from the side wall surface on the second side and preventing the narrowing of the cone of the injected fuel.
Preferably, the outer hole section includes a second outer side wall surface on the second side extending substantially in parallel with the first outer side wall surface.
Thereby, the drilling work of the outer hole section can be simplified.
Also preferably, the outer hole section includes a second outer side wall surface on the second side extending substantially in parallel with the second inner side wall surface.
Thereby, the outer hole section can be drilled in a direction parallel to the inner hole section, and therefore, the drilling work of the outer hole section can be facilitated.
In this invention, preferably, the middle hole section has a larger cross sectional area than the inner hole section, and the outer hole section has a larger cross sectional area than the middle hole section.
Thereby, the separated fuel flow is prevented from colliding with the opposing side wall surface so that the narrowing of the cone of the injected fuel is prevented, atomization of fuel is promoted, and fuel penetration is minimized.
Preferably, the inner hole section consists of a linearly extending hole having a constant circular cross section.
Thereby, the drilling work for the inner hole section of the injection hole can be simplified.
Also preferably, the outer hole section has an outermost part consisting of a linearly extending hole having a constant circular cross section.
Thereby, the drilling work for the outer hole section of the injection hole can be simplified.
According to a preferred embodiment of the present invention, the injection holes are formed in the bottom wall of the nozzle tip portion along a concentric circle relative to the nozzle axial line, and the recess comprises an annular recess concentrically surrounding the injection holes.
Thereby, the recess can be formed in a simple and accurate manner.
Preferably, the bottom wall comprises a conical or dome-shaped wall defining a concave inner surface (31) and a convex outer surface (32), and the recess includes an annular bottom surface (66) orthogonal to the nozzle axial line and a cylindrical side surface (67) extending in parallel with the nozzle axial line.
Thereby, a uniform and favorable distribution of the fuel flow can be achieved so that a favorable atomization of fuel and a reduction in fuel penetration can be achieved in an inexpensive manner.
Thus, the present invention provides a fuel injector that allows further atomization of fuel and further reduction in penetration.